1. Field of the Invention
The present invention relates to SiC substrates and, more particularly, to a method of fabricating SiC substrates.
2. Description of Related Art
Silicon carbide (SiC) is an important wide-bandgap material used for the development and manufacturing of SiC and GaN semiconductor devices. Silicon carbide is used as a lattice-matched substrate material to grow epitaxial layers of SiC and GaN. High planarity of SiC substrates is crucial for achieving high quality epitaxy and subsequent device processing. Bow and warp are two parameters generally used to characterize the deviation from planarity. Bow is determined as the deviation of the wafer center from the plane, while warp is the maximum deviation of the wafer from the plane. Henceforward, warp is used as a more general term describing the wafer deviation from planarity.
In general terms, warp in SiC wafers may have two sources. One is the geometric effects of the fabrication processes. Typically, SiC ingots are sliced into wafers using multi-wire diamond saws. If the travel of the wires is not planar, the sliced wafers could attain cylindrical, wavy or other shapes. The other source of warp is stress in the wafer, which can be bulk stress induced by the growth process, and stress on the wafer surfaces due to the damage induced by slicing and polishing. In practice, though, these factors are usually convoluted. For example, when a stressed crystal is sliced, the stress resolves through slight bending of the wafers; such bending affects the wires yielding warped wafers.
Reduction of wafer warp can be conventionally addressed through optimization of wafer slicing and polishing. This approach does not address the growth-related bulk stress and, therefore, is only moderately effective, especially in the cases when the growth-related stress is a major contributor to the wafer warp.
For the production of n-type SiC substrates, gaseous nitrogen is commonly used as a donor during crystal growth. In order to achieve the desired electrical resistivity of 0.020 Ω·cm and below, the nitrogen concentration in the crystal should be high−1018cm−3 and above. Previously, Rost et al. (H. J. Rost et al., “Influence of Nitrogen Doping on the Properties of 4H-SiC Single Crystals Grown by Physical Vapor Transport”, Journal of Crystal Growth, Vol. 257(1-2), (2003), p. 75) reported that warp in SiC wafers increases with increase in the level of nitrogen doping. While the exact mechanism for such warp increase is not clear, it is believed that it is related to spatially nonuniform stress distribution in the crystal. Therefore, at any given doping level, it is desirable to develop a growth method capable of producing silicon carbide crystals with low and uniform stress.
The most common technique used for the growth of SiC single crystals by sublimation is the technique of Physical Vapor Transport (PVT). A generalized schematic diagram of a PVT growth arrangement is shown in FIG. 1. A sublimation source 2, desirably in the form of polycrystalline SiC grain, is placed at the bottom of a graphite growth crucible 4 and a SiC seed 6 is attached to the container top 8. The loaded container 4 is heated in a manner known in the art to a growth temperature, which is generally between 2000° C. and 2400° C., and a temperature gradient is established between sublimation source 2 and SiC seed 6, whereby the temperature of sublimation source 2 is higher than that of SiC seed 6. At growth temperature sublimation source 2 sublimes and fills the interior of growth container 4 with vapor 10 comprised of silicon- and carbon-carrying volatile molecular species. Driven by the temperature gradient, these species diffuse through the vapor phase in the direction from sublimation source 2 to SIC seed 6. Due to the lower temperature of SiC seed 6, a supersaturation is created causing condensation of vapor 10 on SiC seed 6 and growth of an SiC single crystal boule 12 (shown in phantom) on SiC seed 6.
At high temperatures of sublimation, SiC single crystal boule 12 has a relatively low thermal conductivity. Therefore, the growth of SiC single crystal boule 12 impedes heat dissipation from growth crucible 4 and creates a temperature gradient across SiC single crystal boule 12. This temperature gradient is a source of bulk stress in SIC single crystal boule 12. Furthermore, as the growth progresses, increase in the thickness of SiC single crystal boule 12 causes temperature gradients in SiC single crystal boule 12 to decrease. As a result, the last-to-grow portions of SiC single crystal boule 12 would have a different stress distribution in the bulk of SiC single crystal boule 12 compared to the first-to-grow portions.
In conventional SiC sublimation growth, growth parameters are typically held steady. Therefore, the growth rate is highest at the beginning of growth and then gradually decreases. It is commonly accepted that growth-related stress is influenced by the growth rate. Thus, reduction in the growth rate should contribute to stress nonuniformity throughout SIC single crystal boule 12. The overall result of all of the above factors is a SiC single crystal boule 12 that yields warped wafers.
In U.S. Pat. No. 6,508,880 to Vodakov et al., the inventors carried out a thermal analysis of the SIC growth cell for a specific crystal growth configuration. The performed calculations illustrated the nonuniform nature of the stress distribution in the growing SiC crystal, but did not address the issue of warp in the sliced wafers. No provisions have been made to reduce the spatial stress nonuniformity in the crystal.
U.S. Pat. No. 5,725,658 to Sawada describes a method for reducing warp in III-V semiconductor wafers through a special post-growth thermal treatment applied to the boule. The treatment incorporates a sequence of heating and cooling cycles with interdependent heating and cooling rates. As a result of such thermal cycling, strong transient temperature gradients emerge in the boule bulk leading to spatially more uniform stress distribution. However, application of such treatment to silicon carbide will be inefficient for two reasons. First, the thermal conductivity of SiC is several times higher than that of GaAs or InP; therefore, the temperature gradients produced in SiC using the described thermal cycling will be much lower. Secondly, the technique requires ingots (or boules) with significant length (e.g., 100 mm). Typically, silicon carbide crystals are much shorter (e.g., 10 to 20 mm).
U.S. Pat. No. 5,441,011 to Takahashi et al. describes a method of silicon carbide crystal growth that involves varying the temperature throughout the growth run, with the objective being to obtain SiC crystals of higher quality by reducing the propagation of defects emerging at the interface between the seed and seed-holder. In order to achieve this, a higher initial growth temperature (and thus higher initial growth rate) was used, followed by a gradual temperature decrease. In terms of stress, this invention would make the stress distribution even more nonuniform, making wafer warp worse.
U.S. Pat. No. 6,780,243 to Wang et al. disclosed a SiC growth method aimed at reduction of crystal defects emerging during initial stages of growth. This patent describes a growth process which incorporates the initial stage of very slow growth followed by stepwise reduction of the growth chamber pressure leading to an increased growth rate. Through most of the growth run, the temperature and pressure are held constant. Implementation of this method, however, does not lead to reduction of nonuniformity of stress distribution in the growing crystal.